Protective coating composition



United States Patent 3,287,142 PROTECTIVE COATING COMPOSITION PaulRussell, Jr., Springfield, Pm, assignor to E. I. du Pout de Nemours andCompany, Wilmington, DeL, a corporation of Delaware No Drawing. FiledJuly 22, 1963, Ser. No. 296,494 4 Claims. (Cl. 10614) This inventionrelates to a protective coating composition containing an alkali metalsilicate vehicle pigmented with a metal dust and barium borate and moreparticularly to a cured coating of such a composition.

Protective coating compositions of an alkali metal silicate withpowdered metals, such as zinc and lead, have been used to protect ironand steel structures. These coatings can be cured by exposure to hightemperatures but the more desirable and practical method for largestructures is to cure the coating chemically. US. Patent 2,952,562describes curable coatings which can be used and also methods andmaterials for curing these coatings. However, some of the well knowncoatings have objec- 3,287,142 Patented Nov. 22, 1966 F ICC used.Instead of zinc dust, it is possible to use lead powtionablecharacteristics such as white rusting and apparently do not cure evenlyand uniformly upon application of the chemical curing agent which causesthe coating to blister, flake, and peel in the spots where the coatinghas not been completely penetrated by the curing agent. The faults inthese coatings appear after relatively short periods of time followingapplication, making scrapping, cleaning and repainting of the areasnecessary. There is a definite need for a protective coating compositionwhich has the advantages of the materials of the prior art but which isfree from the aforementioned deficiencies.

This invention provides a new protective coating composition comprisingan alkali metal silicate vehicle, a metal powder such as zinc or leaddust or mixtures of zinc and lead dust, and barium borate. applied to ametal substrate, allowed to dry, and is chemically cured. The resultingcoating provides excellent corrosion resistance to metal substrates,particularly ferrous metal substrates, for extended periods of time,compared to the. prior art material, without blistering, cracking,peeling, checking and with reduced white rusting.

The vehicle used in the coating composition of this invention is a watersoluble alkali metal silicate, water, and a buffering agent. Thepreferred alkali metal silicate is a water soluble sodium silicate witha sodium oxide/silicon dioxide ratio range of /z-FA with the preferredratio being /s:3. However, when thealkali content of the sodium silicateis increased, the drying time also increases, therefore, it is desirableto keep the silicate vehicle at the preferred ratio. Other alkali metalsilicates such as potassium silicate can also be used in this invention,but the sodium silicate gives the better corrosion resistant coating.The most suitable sodium silicate/ water ratio is about 50 parts byweight silicate to 50 parts by weightwater but is operable in the rangeof 90-40 parts sodium silicate with the corresponding l060 parts water.The buffer should be able to maintain thevehicle at about pH 11, sincethis pH level keeps the formation of gases to a minimum when the metaldust is mixed into the vehicle and also reduces the rate at which themixture separates and thereby stabilizes the composition for a longerperiod of time. The preferred buffer is a combination of chromic acidand sodium-phosphate in the ratio of about 2/ 1 and in quantity about 2parts by weight of the vehicle. Other bufiers can also be used such assodium bicarbonate, boric acid and a combination of chromic acid andsodium borate.

The pigment for the coating is preferably a combination of zinc dust andbarium metaborate in a ratio of 85/15 (zinc dust/ barium metaborate);however, ratios in the range from about 75/25 to about 95/5 can also beThis composition is barium metaborate. Small amounts of red lead canalso be used in combination with the barium metaborate and the zincdust. Barium metaborate is the preferred borate pigment. Bariummetaborate coated with sodium silicate to prevent caking and formationof large crystalline hydrates is found to be particularly useful in thisinvention because of its non-hygroscopic nature. Other barium boratessuch as barium orthoborate, barium tetraborate, and

. barium octaborate may also be used on an equivalent weight basis.

The silicate based vehicle and-the pigment are usually combined at thetime the coating composition is to be used since the mixture thickensrapidly and sets in a relatively short time. The mixture range in whichthe coating composition is operable is from -95 parts pigment with thecorresponding 25-5 parts of the silicate vehicle. The preferredcomposition is 91 parts pigment to 9 parts vehicle.

Generally, as soon as the coating composition is mixed, it is applied tothe substrate, which is to be protected by conventional means such asspraying, brushing, dipping, flow coating, and the like. The coatingcomposition is most useful on equipment constructed of metal which isexposed to corrosive conditions such as the interior and exterior ofstorage tanks, above ground piping, furnace stacks, well heads, exteriorhulls of ships, fuel and salt water ballast tanks of vessels, and pumphousings. The applied coating in its dried film state is about 3-4 milsthick but is water soluble and therefore must be cured so that it willbecome hard and insoluble. These coatings can be cured by exposing thecoated surface to high temperatures, ZOO-500 F., for several hours whichis not practical for most applications and in such situations thecoating can be cured by a chemical means. US. Patent 2,952,562 describesvarious chemical curing agents which can be used to cure coatingsproduced with the compositions of this invention.

The chemical curing agent is applied by conventional means to the coatedsubstrate after the coating has dried. The curing agent is allowed tocure the coating for about 8 hours and then the excess agent is washedoff with water if a topcoat of another paint is to be applied. However,the wash step is not necessary if a topcoating is not going to be used.

The curing agent is preferably a solution of an acid salt in an organicvehicle such as alcohol and contains enough water to hydrolyze the acidsalt. The acid salt must be capable of yielding an acid radical whichwill react with the silicate coating to make it hard and insoluble. Thisreaction does occur with the conventional zinc silicate, zinc-leadsilicate, or lead silicate coatings, but the reaction in many instancesapparently only occurs at the surface of the coating leaving thesubsurface unreacted which, in a short time, causes the coating toblister, flake, check and peel. The coating composition of thisinvention permits the coating to cure evenly throughout its thickness(i.e., from the metal substrate to which it is applied to the surface ofthe outer coating).

The curing agent which is particularly useful in this invention is adibutylamine phosphate dissolved in isopropyl alcohol, and water. Inplace of the di-n-butyl amine which is used in the preferred curingsolution to obtain dibutylamine phosphate, it is practical to use anyamine which will react with phosphoric acid or its equivalent to producean amine salt which is soluble in an organic solvent, which is misciblewith water, and which will hydrolyze in solution.

Other acid salts may be substituted for'the phosphoric acid salts suchas magnesium chloride, ammonium phosphate, aluminum sulphate, and otheracid salts of hydro- The sprayed panels are cured by allowing them tostand at normal room conditions about 8 hours and the excess curingagent is then washed from the surface with water.

In all of the coated samples, the films are firmly bonded to thesubstrate indicating adequate adhesion.

EXAMPLE v2 Vehicle Formulation #3 of Example 1 is blended with variouspigments to form the following coating compositions: v

The following vehicle compositions are formulated: n cofiing ffi s g a(all parts are by weight unless otherwise indicated).

97.5 97.5 97.5 97.5 Vehicle formulation #1: Parts 28:? 3%.; sodiumsilicate 68.3 13.5 27.0 E63 WatFf 368.0 368.0 868.0 368.0 7 Sodiumbicarbonate 0.1 20

Boric acid Each of these coating compositions is then sprayed PhosphateT onto a 4 x 12 inch steel test panel and the coating is dried Potassmmchromate about three hours under-normal room atmospheric conm ditionswith the resulting coating being about 3-4 mils 5 thick. The curingagent used in Example 1 is then sprayed Vehicle i iq onto the coatedsamples, the resulting compositions are Sodmm slhcate cured as inExample 1 and the samples are then'washed Water T to remove the excesscuring agent. chrfamlc acld '2 The coated samples are next subjected toa salt (sodium Sodium borate 5 chloride) spray test as described in ASTMB117-62. Sodmm phosphate "7"'" v Five panels-of each group are exposedto the salt spray test and examined after 5, 7, 12 and 20 days. Thetest. loo-0 is briefly described as follows; 7 Vehlcle fTm111 at1n Thecoated panels are placed in a salt spray cabinet. S i i 35 meeting ASTMdesignation B1l7-62; a 5% NaCl fog is Water used and the testtemperature is maintained at 95 F. Chromic acid 1.0 The panels are heldin slots on a rack so that they slant. Sodium phosphate 05 15 from thevertical and the order of placement of the 6 panels on the racks israndomized daily.

100.0 The following table shows the results:

Time

120 Hours 168 Hours 288 Hours 480 Hours Coating I Scattered blisters(fair condi- Scattered blisters (fair condi- Many blistres (poor condi-Failed.

tron). No blisters (excellent condition).

on Scattered blisters (fair coudi- Many blisters (poor condition).

on Ngblisters (excellent condi- N0 blisters (excellent condition). ion

do Do.

0 Coating II N o blisters (excellent condition). Coating III do doCoating IV do do To each of these vehicle formulations a pigment blendof zinc dust/barium metaborate in the ratio of 85/15 is added and thepaint is thoroughly mixed. 91 parts of pigment are used to acorresponding 9 parts of vehicle.

Each of these coating compositions is then sprayed onto a 4 x 12 inchsteel test panel and the'coating is dried about three hours under normalroom atmospheric conditions with the resulting coating being about 3-4mils thick. A curing agent of the following composition is then appliedby spraying:

The above test results show that as the concentration of bariummetaborate in the coating is increased, the

coating provides increased corrosion resistance to steel plates.

I claim:

1. A protective coating composition consisting essentially of 25-5 partsby weight vehicle of aqueous sodium silicate and 75-95 parts by weightpigment of a finely divided zinc powder and barium metaborate, saidpigment having a zinc/barium metaborate weight ratio of about 75/25 to95/5, said sodium silicate solution consisting essentially of about90-40 parts by weight sodium silicate and about 10-60 parts by weightwater.

2. The coating compositionof claim 1 containing 9 parts by weight sodiumsilicate vehicle and 91 parts by weight pigment of zinc dust/bariummetaborate in a g t ratio of about 15. wherein the silicate vehicle hasa pH of about 11, contains about 50 parts by weight sodium silicate andabout 50 parts by Weight water, and has a ratio of Na O:SiO of /2--%.

3. A coated metal substrate in which the coating compn'ses the curedcomposition of claim 1.

4. The product of claim 3 in which the substrate is a ferrous metal.

References Cited by the Examiner UNITED STATES PATENTS 2,576,372 11/1951Toulmin et a1 117-1351 2,576,845 11/1951 McDonald -117135.1

6 Morris et a1. 10614 Morris et a1 10614 Munger et a1 117135.1 Buckmanet a1 106306 Buckman et a1. 23-59 ALEXANDER H. BRODMERKEL, PrimaryExaminer.

0 ALFRED L. LEAVITT, Examiner.

L. HAYES, Assistant Examiner.

1. A PROTECTIVE COATING COMPOSITION CONSISTING ESSENTIALLY OF 25-5 PARTSBY WEIGHT VEHICLE OF AQUEOUS SODIUM SILICATE AND 75-95 PARTS BY WEIGHTPIGMENT OF A FINELY DIVIDED ZINC POWDER AND BARIUM METABORATE, SAIDPIGMENT HAVING A ZINC/BARIUM METABORATE WEIGHT RATIO OF AOUT 75/25 TO95/5, SAID SODIUM SILICATE SOLUTION CONSISTING ESSENTIALLY OF ABOUT90-40 PARTS BY WEIGHT SODIUM SILICATE AND ABOUT 1-60 PARTS BY WEIGHTWATER.